Luminescent rare-earth doped alkaline-earth fluorides have long been known, and have been employed for numerous purposes such as scintillation detectors and laser materials. CaF2 doped with such rare-earth species as Eu+3, Er+3, Tb+3 are well-known compositions. It is well-known that a rare-earth doped alkaline earth fluoride will exhibit luminescence when exposed to ultraviolet light.
Each rare-earth element when incorporated into an alkaline earth host lattice such as CaF2 exhibits a characteristic excitation spectrum; see, for example, FIG. 1 (101), and a characteristic emission or luminescence spectrum that depends upon the excitation wavelength employed; see, for example, FIG. 1 (102). The excitation spectrum is determined by monitoring the luminescence intensity at one wavelength while the specimen is illuminated over a range of wavelengths. The luminescence spectrum is determined by illuminating the specimen at a single wavelength corresponding to a peak in the excitation spectrum and determining the luminescence spectrum by scanning a detector over a range of wavelengths.
As shown in the figures, each such spectrum consists of a plurality of peaks at different wavelengths of light. The wavelengths at which the peaks occur are characteristic of each rare-earth element. No two rare-earth elements exhibit the same excitation or emission spectra; that is, the peaks in their spectra do not in general arise at the same wavelengths. To obtain luminescence, the rare-earth element must be excited by a light source that emits light at a wavelength corresponding to the location of one of the peaks in the excitation spectrum thereof. In general, the peaks in any one spectrum of rare-earth elements differ from one another in height or intensity, these differences in intensity being characteristic of the rare-earth element under particular conditions of measurement. These and related matters are all well-documented in the art. See for example, Martin et al., Atomic Energy Levels—the Rare-Earth Elements, U.S. Department of Commerce, National Bureau of Standards (1978).
Haubold et al., U.S. Published Patent Application 2003/0032192 discloses the use of doped luminescent inorganic compounds for marking goods, such as in use as so-called anti-theft or anti-counterfeiting security markers. Use of rare-earth doped alkaline earth fluorides is not disclosed. Haubold et al., WO 03/052025 specifically discloses printing using the compositions disclosed in Haubold et al., op.cit. No details are provided. Rare-earth doped alkaline earth compositions are not disclosed.
Gardner et al., U.S. Pat. No. 6,861,012, discloses use of phosphorus-based inorganic chelates that are “cropped to” polymer particles employed in ink jet printing inks to provide UV-activated luminescence for marking goods. There is no disclosure of rare-earth doped alkaline earth fluoride compositions.
Ross et al., U.S. Published Patent Application 2005/0143249, disclose the use of rare-earth doped glasses for use in security labels. Disclosed are mixtures of rare-earth doped glasses that give rise to variations in relative emission intensity at pre-selected wavelengths.
Federov et al., Doklady Akademii Nauk. 369(2):217-219, 1999, discloses solid solutions consisting of a series of 10 mm diameter and 50 mm long single crystals of (Ca1−ySry)1−xNdxF2+x grown by the Bridgman-Stockbarger method by crystallization from the melt.
Security marks known in the art generally lack sufficient complexity or encryption to make them difficult to counterfeit themselves. The present invention provides a family of novel rare-earth-doped alkaline earth fluorides, and a process for preparing them, that are characterized by continuously variable luminescence peak intensity ratios, making it extraordinarily difficult to counterfeit security marks having these compositions.